1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, a process cartridge, and an electrophotographic apparatus.
2. Description of the Related Art
Electrophotographic photosensitive members are each requested to provide sensitivity, electrical characteristics, optical characteristics, and a high-quality image free of image defects in accordance with an electrophotographic process to which the electrophotographic photosensitive member is applied. Representative examples of the image defects include image stripes, black dots in a white portion, white dots in a black portion, and ground fogging in the white portion. Further, when any one of the electrophotographic photosensitive members is exposed to light by using the laser diode of a digital copying machine or laser beam printer as a light source, interference fringes that occur owing to the surface profile of the support of the photosensitive member or the non-uniformity of the thickness of the photosensitive member are also included in the examples. A method of suppressing the image defects is, for example, to provide a layer between the photosensitive layer and support of any one of the electrophotographic photosensitive members. The layer between the photosensitive layer and the support is requested to have an electrical blocking function by which the injection of charge from the support is prevented when a voltage is applied to the electrophotographic photosensitive member. This is because of the following reason: the injection of charge from the support is responsible for a reduction in charging performance of the electrophotographic photosensitive member, a reduction in contrast of an image, and, in the case of a reversal developing system, black dots and ground fogging in a white portion described above, thereby reducing the quality of the image.
On the other hand, when the electrical resistance of the layer between the photosensitive layer and the support is excessively high, charge generated in the photosensitive layer resides in the photosensitive layer, thereby causing an increase in residual potential of the electrophotographic photosensitive member or a fluctuation in potential of the electrophotographic photosensitive member due to its repeated use. Therefore, not only the electrical blocking function but also some degree of reduction in electrical resistance of the layer between the photosensitive layer and the support is needed. A method of reducing the electrical resistance of the layer between the photosensitive layer and the support is, for example, to disperse a metal oxide in the layer. The layer between the photosensitive layer and the support disclosed in each of Japanese Patent Application Laid-Open No. 2004-077976, Japanese Patent Application Laid-Open No. 2005-010591, and Japanese Patent Application Laid-Open No. 2005-017470 has the following characteristics: anatase type titanium oxide is incorporated into the layer to reduce the resistance of the layer so that the layer may secure conductivity, and the layer has the electrical blocking function.
However, the layer between the support and the photosensitive layer may be requested to have a hiding function of hiding the defects of the support as well as the conductivity and an electrical barrier characteristic. One known approach to achieving those characteristics is a laminate type layer obtained by: providing a thick layer containing a conductive material on the support; and providing a thin resin layer having the electrical blocking function and free of any conductive material on the thick layer.
In general, the above layer containing a conductive material in the laminate type layer between the photosensitive layer and the support is called a conductive layer, and the layer free of any conductive material in the layer is called an intermediate layer, an undercoating layer, or a barrier layer. A thermosetting resin such as a phenol resin, a polyurethane resin, an epoxy resin, an acrylic resin, or a melamine resin is used in the conductive layer. Investigations were conducted on the use of a polyolefin resin excellent in dielectric characteristic as another resin for use in the conductive layer. However, the polyolefin resin shows poor solubility, and it is not easy to prepare a stable application liquid for the conductive layer, so it has been difficult to use the polyolefin resin as a resin for the conductive layer.
In addition, electrophotographic apparatuses each adopting the following contact charging system have become widespread: a voltage is applied to a charging member (contact charging member) placed to contact an electrophotographic photosensitive member so that the electrophotographic photosensitive member may be charged. Of such systems as described above, the following system is an AC/DC contact charging system: a roller-shaped contact charging member is brought into contact with the surface of the electrophotographic photosensitive member, and a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the member so that the electrophotographic photosensitive member may be charged. In addition, out of such systems as described above, the following system is a DC contact charging system: a voltage formed only of a DC voltage is applied to a contact charging member so that the electrophotographic photosensitive member may be charged.
However, any such contact charging system as described above involves, for example, the following problems: the non-uniformity of charging and the occurrence of the discharge breakdown of the photosensitive member due to direct application of a voltage. The non-uniformity of charging becomes remarkable particularly in the DC contact charging system. The non-uniformity of charging is as follows: portions on the surface of the photosensitive member are not uniformly charged, so stripe-like charging non-uniformity (charging stripes) arises in the direction perpendicular to the direction in which the surface to be charged moves.